The invention relates to a clock system for use in a satellite comunications system. More particularly, the invention relates to such a clock system having an improved stability and reduced clock uncertainty.
SS-TDMA (satellite-switched time division multiple access) is being seriously considered for use in future international satellite systems. Such systems may use TDMA as a principal means for carrying communications information. Also, the use of multiple beams to achieve extensive frequency reuse is expected to become more prevalent.
In such systems, a satellite switch is used to achieve interconnections among the various beams. This allows TDMA traffic bursts transmitted on one beam to the satellite to be routed to any selected receiving location on a designated re-transmission or "down" beam from the satellite as required by a system traffic plan. The beam interconnections used in the switch are programmable and can be changed to optimize traffic flow.
FIG. 1 is a simplified represenatation of an SS-TDMA system with three transmitting stations 2 on the left and three receiving stations 4 on the right. (The transmitting and receiving stations are shown separately; however, they are actually colocated in pairs.) Data transmission occurs in TDMA frame periods with each transmitting station being assigned an "epoch" in the frame during which it can transmit traffic bursts designated for a particular downbeam. Although only one station is shown in each uplink or downlink beam, it should be appreciated that several stations may be located in the same beam and each assigned epochs for transmission in a known manner. Such a satellite switch, shown schematically at the bottom of FIG. 1, routes the traffic bursts to the appropriate down-beams in accordance with a switch state program stored on board the satellite in a memory. In the example illustrated in FIG. 1 three stored switch states, designated I, II, and III, are sequentially used by the switch matrix at the appropriate times during the TDMA frame to achieve the necessary routing. These satellite switch states occur in a frame period which is synchronized to the TDMA frame period. Control of the switch states and the frame period are administered by a satellite switch control earth station 5. In actual operation, many more switch states may be used and a station may be required to transmit a burst more than once per TDMA frame period. Also, the duration of switch states will not necessarily be equal.
In the SS-TDMA system, timing of traffic station burst transmission and reception is governed by the period of the SS-TDMA switch frame. Proper synchronization must be achieved between the SS TDMA switch frame period and the timing of digital transmissions in the terrestrial system which it serves. Otherwise, serious loss of information will occur. Hence, any difference between the requirement to synchronize to the SS-TDMA frame period and the synchronization requirements at the terrestrial system interface must be reconciled.
It has been proposed that international interfaces between digital networks be interconnected by digital buffers designed to permit a time displacement of one primary multiplex frame, for example 125 .mu.s, before a frame slip occurs to permit realignment of the digital transmission signals. It has further proposed that the clock signals governing the timing of each network be controlled to an uncertaintly of .+-.10.sup.-11. This leads to a primary multiplex frame slip of once every 72.34 days at a terrestrial digital interface. This is referred to as plesiochronous operation. If digital satellite links are to be compatible, the digital satellite system must operate within the constraints established for plesiochronous operation.
Thus, for successful operation in an SS-TDMA system, the SS-TDMA clock on board the satellite must exhibit a stability such that the resultant uncertainty does not cause a primary multiplex frame slip more than once every 72.34 days. This means that the uncertainty averaged over 72.34 days must be no greater than .+-.10.sup.-11. Also, other short-term variations such as those due to satellite motion or the control mechanism must be absorbed within the digital satellite system.
A satellite never remains truly stationary; instead it moves about in a "box" centered at its nominal location. For example, for the satellite known as INTELSAT V, this box is defined by maximum variations of .+-.0.1 degrees in the east-west direction for long-term positional drift between position keeping maneuvers, .+-.0.5 degrees in the inclination of the orbital plane relative to the equatorial plane, and an orbital eccentricity of .epsilon.=5.times.10.sup.4. The inclination of the orbit may become as small as .+-.0.1.degree. in tightly controlled satellites. These variational components are illustrated in FIGS. 2(a)-2(d). For the radius of the stationary orbit (41,600 km), the peak-to-peak long term east-west positional variation is 146 km, which occurs over a period of approximately 30 days between position keeping exercises. The maximum peak-to-peak north-south positional variation due to orbital inclinations is 728 km and is a sinusoidal function with a sidereal day period (86,160 sec.). Two components with sidereal day periodicity due to orbital eccentricity occur. An altitude variation (given by .+-. R.epsilon.) is 42 km, and an east-to-west variation (given by .+-.2R.epsilon.) is 84 km.
These variations dictate the size of buffers at earth station terminals needed to absorb path length changes. If the SS-TDMA frame period were established on board by a perfectly stable clock, the buffer at an earth station must only be large enough to absorb the path length change between the station and the satellite. Buffer size is usually expressed in terms of the propagation time of the radio frequency signal over the distance variation involved. The distance variation experienced by an earth station is composed of the projection of satellite position variation on the line of sight to the station. All variations given in the following discussion are peak-to-peak. For a station at the sub satellite point, only the altitude variation is seen; hence, the buffer must be large enough to accommodate a sidereal day period distance variation of 42 km or a time variation of 140 .mu.s. For a station located at the horizon on the equator, the altitude variation is reduced to 138 .mu.s, but a long-term east-west variational component of 74 .mu.s and a sidereal day east-west component of 42.5 .mu.s appear. In this case, the north-south variation is negligible.
For a station located at the horizon in a plane including the axis of the earth's rotation and the satellite, the north-south component is maximum and results in a sidereal day period variation of 375 .mu.s and an altitude sidereal day period variation of 138 .mu.s. If the two components are in-phase, the peak-to-peak sidereal day variation becomes 513 .mu.s. If the orbital plane inclination angle variation is reduced to .+-.0.1 degrees, the latter north-south contribution is reduced to 74 .mu.s and the combination of both components becomes 212 .mu.s. The maximum variation does not occur for this case, but in a plane 81 degrees relative to the one defined above, the maximum variation is 520 .mu.s for a .+-.0.5 degrees orbital plane. If the orbital plane variation is reduced to .+-.0.1 degrees, the maximum variation occurs for a station at the horizon in a plane 45 degrees. In this case, the maximum variation is 244 .mu.s.
Buffers at the earth terminals provided to accommodate the variations given above will eliminate the consequences of satellite motion from the clock stability problem. Then, the only remaining consideration is the uncertainty in the onboard clock. The onboard clock uncertainty will effectively be transferred to all satellite system terrestrial interfaces. Thus, the onboard satellite clock is literally a worldwide timing reference.
Due to the weight and size restrictions, typically a satellite-born clock will have an uncertainty of no better than .+-.10.sup.-9 per day. This is unacceptable for future proposed satellite communication systems. Since provision of an isolated reference clock on board the satellite with the desired small uncertainty is not economically conceivable with present technology as an atomic clock must be used to achieve the desired uncertainty, an alternate means of achieving the necessary level of uncertainty must be provided.
The onboard components of a previously proposed SS-TDMA switch as shown in FIG. 3 include a dynamic microwave switch matrix 14, a distribution control unit 12, and a timing unit or clock 10. The distribution control unit is programmed to provide a cyclic sequence of connections to the switch matrix 14 to execute desired beam interconnections. The timing of the cyclic sequence within the switch matrix 14 is controlled by the onboard clock 10.
The switch sequence may be considered to consist of individual stationary connection patterns called switch states. FIGS. 4(a) and 4(b) illustrate two ways of depicting switch states. FIG. 4(a) shows in crossbar matrix notation connections from various up-beams to down-beams indicated by dots at row-column intersections, and FIG. 4(b) shows in columnar notation connections from various up-beams identified in the terms of the down-beams to which connections are made.
Cascaded switch states form a switch state sequence such as the one shown in FIG. 5 which uses columnar notation. The switch state sequence is repeated at the SS-TDMA frame rate. Each frame is divided into a synchronization field, used for acquisition and synchronization of the satellite switch and for distribution of the TDMA reference bursts, and a traffic field, which provides the up-beam to down-beam connections needed to carry the traffic. This arrangement is shown on a 6.times.6 beam system as an example. FIG. 5 also shows a typical structure for the synchronization field. State I consists of a short state that provides loopback connections to the congruent beam to accomplish acquisition and synchronization by means of an acquisition and synchronization unit. This is called the synchronization window. State II is a short open connection state needed to terminate State I loopback connections, and State III is a reference burst distribution state. Other switch states comprise the traffic field. The following discussion is related to State I, which provides the loopback synchronization window connection for acquisition and synchronization of the satellite switch frame.
A block diagram of a known acquisition and synchronization unit 19 is shown in the lower portion of FIG. 3. It is located at one of the earth stations shown in FIG. 1 which is called a reference station. In this unit, a phase error measurement circuit 20 receivess signals picked up and amplified by an antenna-receiver 18. The operation of the phase error measurement circuit 20 will be explained in detail below. The digital output from the phase error measurement circuit 20 is fed to the control input of a programmable divider 26 and to the data input of an accumulator 22. The accumulator 22 is clocked periodically, e.g., once each sidereal day, to compensate for Doppler shift and voltage-controlled-oscillator (VCO) drift. The output of the accumulator 22 is fed to the control input of a voltage-controlled oscillator 28, the output of which is coupled to the clock input of the programmable divider 26.
The output pulses produced by the programmable divider 26 trigger a burst transmitter 24 which emits test or metering bursts which are transmitted to the satellite. The metering burst, which is fixed in format and which will be explained in further detail below, is used to carry out a measurement of the difference in time alignment between the bursts transmitted by the earth station and the synchronization windows set by the satellite.
The acquisition and synchronization unit 19 establishes and maintains synchronization between TDMA reference station bursts and the SS-TDMA frame, the timing of which is determined by the onboard clock. It has two modes of operation, acquisition and synchronization. In the acquisition mode, a special low-power, two-tone FSK-modulated burst is transmitted to locate the synchronization window with coarse accuracy. In the synchronization mode, a special metering burst is transmitted which permits accurate metering of the difference between the burst location and the trailing edge of the synchronization window. FIG. 6 illustrates the relationship of the metering burst to the trailing edge of the synchronization window.
The control function of the acquisition and synchronization unit 19 is organized so that the center of the metering burst is maintained at the trailing edge transition of the synchronization window. FIG. 7 illustrates the method of accomplishing the measurement. It is assumed that each metering segment contains 16 QPSK symbols and that each symbol carries two information bits. A stored bit pattern is used to generate the metering segment. A metering burst is transmitted and returned to the same controlling earth terminal. When the burst is returned after passage through the satellite switch, the received pattern is compared with the stored pattern and the number of correctly received bits counted. if the burst has been truncated precisely at midpoint, the first eight symbols will contribute 16 correct counts, while the last eight (which have been eliminated) will have states determined by random occurrence, and therefore will contribute an average of only eight correct counts. Thus, when an average of 24 correct counts are received for each metering segment, the center of the metering burst is centered on the trailing edge of the synchronization window. The trailing edge intersection may also be determined by the bit count transition of adjacent symbol positions.
Because a large number of samples are needed to acheive sufficient accuracy, a set of L metering bursts is used in the averaging process. Typically, L will be 32 or 64. When all factors involved are accounted for, including channel error rate, turn-off transitions, and statistical variance due to sample size, the accuracy of the method is .+-.1 symbol period for a 120-Mbit/s TDMA transmission rate.
The operation of the acquisition and synchronization unit shown in FIG. 3 will now be described. It is assumed that acquisition has taken place and that the acquisition and synchronization unit is maintaining the location of the metering segment of its burst transmission. Each TDMA frame, a burst is transmitted using timing derived from the local clock of the acquisition and synchronization unit using a programmable divider. Using the most recent set of L metering bursts, the phase error measuring circuit measures the displacement between the metering burst center and the synchronization processing. The result of this measurement, which is designated x.sub.i, is supplied to both the programmable divider 26 and to an accumulator 22.
The programmable divider 26 immediately makes a displacement correction in the amount x.sub.i to realign its burst transmission in an attempt to reduce the displacement error to zero. Values of x.sub.i can be updated with a period no less than the round-trip propagation time to the satellite plus the duration of the sample smoothing interval.
The value of x.sub.i is also accumulated by the accumulator 22 over an interval of N observations of x.sub.i and used to generate long-term rate corrections for the voltage-controlled oscillator 28. The value of N is selected so that clock corrections occur at, for example, 20 second intervals. This action causes the local voltage-controlled oscillator 28 to track the onboard oscillator drift as well as the frequency variation due to Doppler effects. Thus, the acquisition and synchronization unit generates burst transmission timing at the earth station which aligns the arrival times of the metering burst center at the satellite precisely at the trailing edge of the synchronization window. This timing is also available to generate the reference bursts needed to control the TDMA network.
The acquisition and synchronization unit operation discussed thus far provides only a means of tracking the onboard oscillator of the satellite. Hence, onboard oscillator uncertainty is transferred to the entire TDMA network. This would be sufficient for plesiochronous operation only if the onboard oscillator inherently possessed the required .+-.10.sup.-11 uncertainty which, as explained above, is not presently technically feasible.
Accordingly, it is an object of the present invention to provide a satellite clock system in which it is not necessary to provide an onboard clock in a satellite having an extremely small uncertainty and in which the overall system uncertainty is considerably better than that of the onboard clock of the satellite.